“This star is smaller, and likely colder than many of the gas giant exoplanets that have so far been identified,” said Alexander Boetticher of the University of Cambridge, the lead author of the study. “It might sound incredible, but finding a star can at times be harder than finding a planet.”

Stars are formed from clouds of gas that collapse under gravity and emit energy—usually in the form of light and heat—due to nuclear reactions that take place at their cores. Planets orbit around stars, while an exoplanet is a planet outside our solar system.

It is likely that the newly-identified star is as small as a star can possibly become, according to the researchers. Stars require a certain level of mass to enable hydrogen fusion to take place at their cores. Nuclear fusion of hydrogen atoms to form helium is the process by which stars, including the Sun, release energy, which keeps them from collapsing under their own weight.

Were the star in question to be any smaller, it would not likely have the requisite mass to sustain the nuclear fusion. It would have instead become a brown dwarf, celestial objects that are bigger than giant planets but smaller than the smallest stars and emit infrared radiation, said Boetticher.

The Trappist-1 solar system. Three of the planets (e, f and g) are located firmly within the "habitable zone." NASA/JPL-Caltech

Small and dim stars are key to discovering Earth-like planets that may have liquid water on their surface, an essential criteria for the possibility of life.

Experts have found that TRAPPIST-1, a dwarf star that is slightly larger than Jupiter, is being orbited by seven planets similar to size in Earth. Three of these planets are in the so-called “habitable zone” of the star, where temperatures are neither too hot nor too cold to preclude the existence of liquid water. The newly-identified star, EBLM J0555-57Ab, has a similar estimated mass to TRAPPIST-1 but a radius that is almost a third smaller.

“The smallest stars provide optimal conditions for the discovery of Earth-like planets, and for the remote exploration of their atmospheres,” said the study’s co-author, Amaury Triaud, senior researcher at Cambridge’s Institute of Astronomy. “However, before we can study planets, we absolutely need to understand their star; this is fundamental.”

Data from NASA’s Kepler spacecraft has shown that there could be as many as 40 billion Earth analogs—or planets that are similar in size and conditions to Earth, and therefore might sustain life—in the galaxy, which includes our solar systems and others outside it. When looking for planets that could sustain life, astronomers usually look for candidates that are similar to Earth, such as ones that may possess liquid water or are of a similar size.

The newly-measured planet was found by WASP (Wide Angle Search for Planets), an experiment co-run by the British universities of Keele, Warwick, Leicester and St. Andrews. It was measured as it passed in front of its parent star; this process meant that the parent star became dimmer as the smaller star orbited it.